1. Field of the Invention
The present invention is generally concerned with unifocal ophthalmic lenses, that is to say constant power lenses, whether they are convergent (positive power) lenses or divergent (negative power) lenses.
It is more particularly directed to lenses in which the convex front surface and the concave rear surface are both part-spherical.
2. Description of the prior art
These lenses have the advantage of being relatively easy to machine.
They give rise to two problems, however.
The first relates to the fact that for relatively high powers their maximum or critical thickness, which is the thickness at the center for a positive power or the thickness at the periphery for a negative power, is relatively high, which is detrimental from the esthetic point of view and, given their resulting weight, from the user comfort point of view.
The second problem relates to the fact that, especially for higher powers, abberations and in particular astigmatism and field curvature abberations become increasingly important as the user's axis of vision moves away from the optical axis of the lens.
To minimize such abberations an aspherical surface is usually employed for at least one surface of the lens, at the cost of complicated machining thereof.
Also, the refractive index of the material contituting ophthalmic lenses is usually uniform.
In the case of ophthalmic lenses with part-spherical concave and convex surfaces the power is then geometrically determined by the refractive index, the radius of curvature of the surfaces and the thickness at the center.
However, it has already been proposed to vary the refractive index of an ophthalmic lens radially from its optical axis to its periphery.
This is the case, for example, in British Patent No 1 571 930 and also in published French patent application No 2 599 157.
In both cases this is essentially to correct abberations.
In more precise terms, in British patent No 1 571 930, in which the refractive index varies in a quasi linear manner, the preferential arrangement is such that, given this variation in the refractive index, the concave and convex surfaces of the lens concerned are still effectively part-spherical.
In published French patent application No 2 599 157, in which the concave and convex surfaces are necessarily part-spherical, it is in principle a question of minimizing the critical thickness of the lens.
However, the examples described in this document show that the refractive index does not necessarily vary in any significant manner in the central part of the lens, which is the most used part, and most importantly that the optical power of the lens does not differ in any significant way from its geometrical power.
The present invention is based on the fact, not previously demonstrated, that by appropriately varying the refractive index it is possible to modify significantly the optical power of a lens relative to its geometrical power, while achieving satisfactory correction of astigmatism and field curvature.
It is directed to the manufacture of unifocal ophthalmic lenses which advantageously have accurately part-spherical concave and convex surfaces and are therefore easy to machine, which achieve sufficient correction of astigmatism and field curvature aberrations, and which advantageously have a reduced maximum thickness.